Mechanism of action of AZD0865, a K+-competitive inhibitor of gastric H+,K+-ATPase

AZD0865 is a member of a drug class that inhibits gastric H+,K+-ATPase by K+-competitive binding. The objective of these experiments was to characterize the mechanism of action, selectivity and inhibitory potency of AZD0865 in vitro. In porcine ion-leaky vesicles at pH 7.4, AZD0865 concentration-dependently inhibited K+-stimulated H+,K+-ATPase activity (IC50 1.0 ± 0.2 μM) but was more potent at pH 6.4 (IC50 0.13 ± 0.01 μM). The IC50 values for a permanent cation analogue, AR-H070091, were 11 ± 1.2 μM at pH 7.4 and 16 ± 1.8 μM at pH 6.4. These results suggest that the protonated form of AZD0865 inhibits H+,K+-ATPase. In ion-tight vesicles, AZD0865 inhibited H+,K+-ATPase more potently (IC50 6.9 ± 0.4 nM) than in ion-leaky vesicles, suggesting a luminal site of action. AZD0865 inhibited acid formation in histamine- or dibutyryl-cAMP-stimulated rabbit gastric glands (IC50 0.28 ± 0.01 and 0.26 ± 0.003 μM, respectively). In ion-leaky vesicles at pH 7.4, AZD0865 (3 μM) immediately inhibited H+,K+-ATPase activity by 88 ± 1%. Immediately after a 10-fold dilution H+,K+-ATPase inhibition was 41%, indicating reversible binding of AZD0865 to gastric H+,K+-ATPase. In contrast to omeprazole, AZD0865 inhibited H+,K+-ATPase activity in a K+-competitive manner (Ki 46 ± 3 nM). AZD0865 inhibited the process of cation occlusion concentration-dependently (IC50 1.7 ± 0.06 μM). At 100 μM, AZD0865 reduced porcine renal Na+,K+-ATPase activity by 9 ± 2%, demonstrating a high selectivity for H+,K+-ATPase. Thus, AZD0865 potently, K+-competitively, and selectively inhibits gastric H+,K+-ATPase activity and acid formation in vitro, with a fast onset of effect.